Recently, processing technologies of the semiconductor devices are developed rapidly, active cell isolation technologies for isolating active cells become one kind of very important technologies in view of micronizing (i.e., reducing the size or the design rule of) the semiconductor device.
In general, of the known semiconductor device active cell isolation technologies, LOCOS (local oxidation of silicon) has been the major technology, in which a thick oxide film is grown selectively on a substrate to form a series of active cell isolation bodies.
However, LOCOS has a disadvantage in that a width of the active cell isolation region can not be reduced below a certain level due to lateral diffusion of the active cell isolation film and the known bird's beak phenomenon. More recently, some new technologies for overcoming the disadvantage of the LOCOS technology such as “a trench technology”, “an STI (Shallow Trench Isolation) technology”, “an air gap STI technology”, etc., have been developed and used widely.
Referring to FIG. 1A, in the known STI technology, for an example, the air gap STI technology, the active cell isolation body is formed by using a sacrificial oxide film 11 and a sacrificial nitride film 12 on a semiconductor substrate 1 and patterning the sacrificial oxide film 11 and the sacrificial nitride film 12 with a photoresist pattern 13, forming a trench 2 in an active cell isolation region FR of the semiconductor substrate 1 by using the patterned sacrificial oxide film 11 and the sacrificial nitride film 12 as mask, and growing an oxide film 3 on an etched surface of the trench 2, filling a filler material in the trench 2, and forming a cap layer 5 on the semiconductor substrate 1 inclusive of the filler material 4 as shown in FIG. 1B, evaporating the filler material 4 from the trench 2, to form a series of gaps ‘g’ in the trench 2, and depositing a thick insulating layer 6a on the cap layer 5 inclusive of the trench 2 as shown in FIG. 1C, planarizing the insulating layer 6a to form an active cell isolation film 6 on the trench 2 as shown in FIG. 1D, and removing the sacrificial oxide film 11 and the sacrificial nitride film 12 as shown in FIG. 1E. The active cell isolation body 7 formed in this manner serves to insulate active cells securely when a series of the active cells are subsequently fabricated on the active region AR.
In the known STI technology, a scale of the trench 2 is an important factor in determining a quality of the semiconductor device. This is because, if the trench 2 is a too small scale, which weakens a device isolating function of the active cell isolation body 7 significantly, there can be a serious problem of unnecessary interferences caused between the active cells on the active regions AR of the semiconductor substrate 1. However, even if the reduction of the scale of the trench 2 affects the device isolating function seriously, the scale of the trench 2 can not be increased without a plan. This is because if the scale of the trench 2 is enlarged too much, an effective scale of the active region AR can be reduced significantly due to an influence of the enlargement, resulting in a serious difficulty during subsequent fabrication of the active cells.
Even if it is known that the reduction of scale of the trench 2 affects significantly the active cell isolation function, an actual counter measure for the reduction of the effective scale down of the active region AR has not been provided.
Of course, if the scale of the trench 2 is reduced and no measure is taken, the active cells formed on the active region AR contribute to the unnecessary degradation of performance due to failure of an insulating function of the active cell isolation body 7. In the meantime, as described before, with the known STI technology for forming the air gap ‘g’, it is required to fill the trench 2 with the filler material 4 and evaporate the filler material 4. However, because the filler material 4, which may be amorphous carbon, silica gel, silic acid polybutadiene, and so on, are expensive and can be difficult to work with because these materials are required to be vaporized at a temperature lower than surrounding oxide films, as far as use of the filler material 4 is not excluded perfectly, which increases a total cost for fabricating the semiconductor.